Hydro-transport, or the transport of particulate materials in slurry form, usually entrained in a flow of water, is used in the mining and mineral industries. The material is fluidized in batches and conveyed along pipelines. Such processes can be used in many other industries where solids are moved, such as filtration, water softening, catalyst regeneration and grit or shot blasting systems.
Fluidizing transport systems in which the material to be transported is fluidized in a pressure vessel and then discharged from the vessel under fluid pressure are disclosed in U.S. Pat. No. 4,978,251, U.S. Pat. No. 4,952,099, U.S. Pat. No. 5,853,266 and WO2007015091. Such systems are capable of transporting slurries from the pressure vessel at much higher solids concentrations than a traditional slurry pump, are more energy efficient, and have the advantage that no moving parts come into contact with the solids.
The problem with these fluidizing systems is that, owing to the high concentrations of solids and the speed of transfer of the solids from the pressure vessel into the transport pipeline, the vessel must be replenished with solids rapidly to keep up with the discharge. This requires the vessel to be depressurized and solids loaded, normally under gravity, into the top of the vessel. In order to load quickly, the pressure vessel has a fill valve or port which is generally a minimum 4 times larger than the discharge pipe. The valve must be quick-acting, pressure-retaining and solids tolerant, and be capable of undergoing several hundred thousand operating cycles. Valves on large-scale systems may be 1 meter diameter or greater.
The above constraints are generally beyond the normal operating limits of many valves. A valve capable of this duty is generally large and expensive making the use of fluidizing transport systems uneconomical.
Hard seat ball valves or plug valves can be used, but the operating conditions and abrasive nature of the solids cause rapid wear of valve seats and seals, necessitating a high level of maintenance.
Owing to the size of the valves, the actuators which drive them are equally large and require frequent maintenance. The plant must be taken out of service during maintenance work, resulting in down time and lost production, together with associated costs.
Pinch valves are known to be solids tolerant and can be pneumatic or mechanically operated; however, they do not have a high pressure capability, have a limited size, and so are not suitable except on low pressure low volume systems.
Knife valves, whilst generally smaller in depth, can be used but again require an actuator and have a limited operating pressure. The seals employed in knife valves are generally not suitable for continuous cyclic operation.
Fluidization hydro-transport systems rely generally on a flooded pressure vessel and a solids accumulation hopper located above, and operate as follows. While the hopper is being loaded with solids, the standing fluid within the hopper is displaced from the hopper via an overflow pipe. When the hopper has been filled with solids, water will be present in the hopper within the voidage of the solids. The transport fluid is usually water. Consequently, it will be referred to as water in this specification but it will be appreciated that other fluids, usually liquids can be used.
Water as the transport fluid is fed into a fluidizing unit within the pressure vessel, the fluid displacing the solids present in the pressure vessel until all the solids are discharged. This leaves the vessel full of water.
When the pressure vessel is empty of solids the valve between the hopper and vessel is opened and solids fall by gravity into the pressure vessel. Water within the vessel is displaced by the higher density solids entering the vessel. This results in solids falling through a counter-flow of water rising through the valve opening from the vessel, which can hold up or restrict the loading of solids.
To overcome this, a separate displacement or bypass line complete with a simple shut-off valve can be provided, extending from the top of the vessel to the top of the hopper. This allows the majority of displaced water to bypass the large solids loading valve. This reduces the counter-flow of water through the valve to allow the solids to fall into the vessel unhindered which speeds up the loading time. In all cases the hopper and vessel are in a flooded state either before or after loading.
Once the vessel is loaded, the valves around the vessel are closed, and feed to the fluidizing head is opened. This feeds water into the vessel via the fluidizing head and brings the vessel up to working pressure, normally within 1 or 2 seconds. The solids then discharge as slurry into a transport pipeline.